Recently, LSIs (Large Scale Integration circuits) for processing digital images have a significant problem of insufficient memory bandwidth (the number of accesses per unit time) resulting from an increase in the size of images to be processed.
Now, this problem will be explained with reference to FIG. 1, using “color key transparency”, which is one image blending method, as an example. “Color key transparency” is the image blending method to blend two images by removing only a specific color referred to as “color key” from an original image and fitting another image in the original image. It is possible to set any colors as a color key.
FIG. 1 shows a case in which image C is created by blending image A and image B stored in a memory. Color key data is set in image A. Processor (CPU: Central Processing Unit) 1 creates image A, that is, sets color key data for image A. Image A with set color key data is written to memory 4 through memory controller 3. When blending image A and image B, image processing section 2 accesses memory 4 through memory controller 3 and reads image A and image B from memory 4. Then, image processing section 2 allows image B to appear to draw image B when color key data of image A is read, but does not allow to image B to appear to draw image A when color key data image A is not read. By this means, it is possible to obtain composite image C.
As described above, with conventional color key transparency, even color key data is read from memory 4. However, color key data serves no purpose as data, so that the memory bandwidth for this color key data is desired to be reduced from the perspective of reduction in wasteful memory accesses and consequently a speed-up in memory accesses.
Conventionally, there are technologies described, for example, in patent document 1 and patent document 2, as a technology to reduce memory bandwidth (the number of accesses per unit time).
The technology described in patent document 1 aims to reduce the number of memory accesses by preventing write processing in the process of drawing in an image memory, including reading/modifying/writing, if the result after modifying is the same as read data.
Meanwhile, the technology of patent document 2 aims to reduce the number of memory accesses for unnecessary write processing in a facsimile and so forth that compresses continuing identical image data by run-length coding: by preventing generation of processing other than write processing to the first address in continuing image data when identical image data continue while clearing the display memory using a predetermined value in advance; and returning previous image data when it is possible to read the clear value.    Patent Document 1: Japanese Patent Application Laid-Open No. 5-266177    Patent Document 2: Japanese Patent Application Laid-Open No. 1-126687